Bacteria are organisms

Bacteria Are Simple Cells

Bacteria can live in many environments where other organisms cannot survive. These environments include icy polar lands, boiling hot springs, deep ocean waters and even the air you breathe.

Some bacteria live independently, some in close, permanent association with organisms of other species. Such conditions are called symbiosis. Three forms of symbiosis occur. In mutualism both organisms – the bacterium and its host – benefit. In commensalism one organism benefits and the other is neither helped nor harmed. In parasitism one organism benefits and the other is harmed.

Bacteria are among the simplest and smallest living things. They are tiny monerans that have no separate nucleus. A single chromosome of DNA is found in their cytoplasm. Bacteria have no mitochondria or endoplasmic reticulum. Their cytoplasm is surrounded by a cell membrane. Enzymes that help bacteria release energy from food are found along the cell membrane. It provides shape and can keep bacteria from bursting. Some bacteria are also surrounded by simply capsules that give them extra protection.

Most bacteria have one of three basic shapes: rod, round, or spiral. The rod-shaped ones are called bacilli; the round ones, cocci; and the spiral ones, spirilla.

Some bacteria have long thin extensions called flagella; A bacterium may have a single flagellum or numerous flagella, all over its cell surface. Pili are extensions similar to flagella, but are shorter, thinner, and more numerous. Bacteria use pili to attach themselves to a source of food or oxygen or to another bacterium.

Not all bacterial cells are capable of movement. Those that move are called motile and those that do not move are called nonmotile.

Bacterial nutrition and metabolism

Bacteria are extremely versatile metabolically. Some are autotrophic – that is, they produce their own food. The most familiar autotrophic organisms are those that are photosynthetic. Some autotrophic bacteria derive their energy from sunlight, others from the breakdown of inorganic chemicals – a process called chemosynthesis.

Most bacteria are heterotrophs, depending on autotrophs for food. Heterotrophs may ingest other organisms, as most members of the animal kingdom do; they may be saprobes, procuring organic molecules from dead organic matter or they may be parasites, using organic molecules from living organisms.

Respiration

Some bacteria need oxygen to carry out respiration. Others use oxygen when it is available. Some doesn’t need oxygen at all, and in fact die in the presence of oxygen.

Bacteria that require oxygen to live are called obligate aerobes. Obligate aerobes generally live where there is an ample supply of oxygen, such as in the air or in loose soil. Mycobacterium tuberculosis, is an obligate aerobe that can live in the lungs of human beings.

Bacteria that cannot live in the presence of oxygen are called obligate anaerobes. These organisms are found where there is little or no oxygen, such as deep in the soil or in mud at the bottom of lakes. Methanogens are obligate anaerobes. An obligate anaerobe called Clostridium botulinum causes a dangerous type of food poisoning known as botulism. If canned food is improperly prepared, botulism bacteria may begin to grow in the almost oxygen-free environment inside the can or jar.

The third type of bacteria, called facultative anaerobes, can grow with or without oxygen. The most common bacterium in the human digestive tract is a facultative anaerobe.

Growth. Reproduction and genetic transfer

Bacterial growth usually refers to an increase in the number of bacteria, rather then to an increase in its size. A colony is a large group of bacteria, such as that grown on a nutrient plate in a laboratory. All the members of colony are descendants of a single bacterium. All bacteria need food and water and many need oxygen. Other factors that influence growth are temperature, sunlight and chemicals.

Under ideal conditions, some bacteria can reproduce every 20 to 30 minutes. Bacteria usually reproduce through binary fissions, or splitting in two. Each new cell is exactly like the parent cell. Mutations, the result of errors or changes in the genetic code, account in part for the extraordinary ability of bacteria to adjust to differing conditions (Tabl. 3.1).

Genetic material in some cases is transferred from one bacterium to another, resulting in genetic recombination. When the bacterium later divides, it passes on its new genes to the daughter cell. In this way, resistance to antibiotics can be transferred from one of bacteria to another.

In a process called conjugation, a donor bacterium transfers genetic material to an acceptor bacterium through direct contact.

Transformation process transfers genetic material from a dead strain, or genetic type, of bacteria to a live strain.

Date: 2014-12-22; view: 913